Crypto-oriented SIMD abstractions

Design: - interface as close to RFC2366/packed_simd as practical - pluggable backends

Backends supported: - ppv_null: Emulated SIMD. Safe, portable. - ppv_lite86: x86 implementation using coresimd intrinsics, stable and fast to compile. - packed_simd: Support for future compatibility--can probably replace ppv_lite when it's eventually stable; in the meantime, offers an unstable SIMD backend for platforms ppv_lite doesn't support.

Experimental status

Initially I'm adding functionality as needed for my crypto implementations, so there will be random gaps in the interface. Eventually I will round out the feature set and define the available functionality in traits, to ensure that the backends support the same functionality.